Signals are transferred across printed circuit boards (PCB's) typically using PCB interconnects in the form of linearly disposed conductive transmission lines, such as transmission lines routed according to a standard Manhattan method. A transfer rate of a signal across a transmission line is among other things a function of an effective dielectric constant of the board or panel underlying the transmission line's trajectory. Conventional panels may be made from a thermoset resin such as an epoxy filled with a fibrous reinforcement such as glass fiber to form a board. An example of such a board includes FR4, a schematic illustration of which is provided as panel 101 in FIG. 1. FIG. 1 shows a portion 100 of a printed circuit board including an FR4 panel 101 and two transmission lines 110 and 112 on the panel 101 according to the prior art.
As seen in FIG. 1, as transmission line, such as lines 110 and 112, advance on a given panel, they each cross over areas of the panel exhibiting varying dielectric constants by virtue of spatially varying woven fiber densities within the panel. An effective dielectric constant of the panel underlying the transmission line's trajectory would therefore in effect correspond to an averaging of the varying dielectric constants. For a given bus layout design, bus lines typically run over paths on the panel that may substantially different from one another. As seen in FIG. 1, the two lines 110 and 112 are shown as having trajectories that extend over substantially different physical regions of the panel, the two lines thus having widely varying signal transfer rates. Where differential signals are transmitted through the transmission lines of a bus, the localized spatial variations in PCB panels of effective dielectric constants create a phase skew in the signals being transmitted. This phase skew converts differential signals to common mode as the signals propagate along the PCB transmission lines, the above phenomenon being commonly referred to as “mode conversion.” As transfer rate requirements of PCB transmission lines increase into the multi Gb/s range, the allowable timing tolerance between different lines on the PCB panel shrink correspondingly, up to a point where resulting local variations in dielectric constant cause appreciable voltage and timing noise, and phase skews that dramatically reduce signaling data rates, leaving an insufficient margin for other effects, such as transmitter jitter, ISI, and crosstalk, to name just a few.
The prior art mitigates the problem of phase skew as noted above according to a number of different options. Some of these options are discussed below.
One option involves routing predetermined pairs of transmission lines at an angle relative to the weave direction to average out dielectric constant variations by constantly changing the relative position of the transmission lines relative to the bundles. For example, routing at a 45 degree angle can reduce phase skew. Another option entails using design rules that place a maximum length limit on pairs of transmission lines beyond which the pair must shift to a different track. By offsetting the routing tracks by one half of the weave pitch, mode conversion can be cancelled by matching the trace lengths for each track. A third option mitigates mode conversion by rotating the image of the tracks relative to the entire panel, such as by 3 to 5 degrees. However, the above option generates a fair amount of waste with respect to unused portions of the board onto which the rotated image is not transferred.
All of the above options are disadvantageous to the extent that they complicate a fabrication of the PCB by adding fabrication stages, decreasing fabrication efficiency, and adding cost. In addition, the option of routing predetermined pairs of transmission lines at an angle relative to the weave direction is feasible sometimes only by adding panel layers in order to compensate for a possible shortage of panel surface area as brought about as a result of an angling of selected transmission lines.
Other mode conversion mitigation options include the use of fiber reinforced materials that use a tighter weave to reduce the gap between bundles, an elimination of the fiberglass cloth using only the resin, or the use of chopped fiberglass to randomize dielectric constant variations. However, disadvantageously the latter options are either expensive to implement and/or not applicable to the entire extent of the panel being used.
The prior art has failed to provide a reliable and cost-effective method of mitigating phase skew in differential signals caused by localized spatial variations in the dielectric constants of PCB panels.